Recently, there is proposed an apparatus including a probe having a vibrator which emits an ultrasonic wave toward a cornea of an examinee's eye and a sensor for detecting the ultrasonic wave reflected by the cornea to measure the IOP of the eye in non-contact manner (see Patent Literature 1).
In the case where output signals from the probe are processed to obtain the IOP, conventionally, the IOP is determined based on a frequency shift amount of a reflected wave with respect to an incident wave.